This invention relates to thrust bearings and more particularly hydrodynamic thrust bearings adapted to function with very thin lubricant films.
Thrust bearings form important elements of many machines, including many machine tools. Currently most thrust bearings of the hydrodynamic type have two drawbacks: excessive friction and wear. Ball or roller thrust bearings have some limitations with respect to cost, durability, tolerance capability and load capacity. Research at the inventor's laboratory has indicated that the loadbearing potential of hydrodynamic bearings has been greatly underestimated because the effect of contact (Hertzian) deformations, which alter the shape of the sliding parts under load in a highly desirable way, has not been adequately understood. Enough analytical understanding of the relevant effects now exists to produce very effective and inexpensive hydrodynamic thrust bearings.
Hydrodynamic fluid mechanics is so ubiquitous in technology that it is easy to forget what a fine scale phenomenon it is. Very thin lubricant films of oil or grease are involved, often less than 0.0001" thick. Convergent angles between sliding parts for best load capacity are remarkably small, frequently 0.003"/ft. or less. The physics of hydrodynamic fluid mechanics happen on very fine scales, and involve equations which are relativey difficult to manipulate, even for professionals.
In the current thrust bearing, the scale of the convergent surfaces which produce the hydrodynamic action involves a peak to valley wave height of a few tenths of a thousandth of an inch to a few thousandths of an inch. These very shallow radial waves are what is required for good hydrodynamic function. A number of methods of making disk-shaped thrust bearings with this degree of surface waviness are disclosed in this case. But a significant consequence of the very fine scale geometry of the thrust bearing or any other hydrodynamic bearings requires emphasis. This consequence is the major contribution which deformation makes to the performance of any hydrodynamic bearing.
At the scales which matter for hydrodynamic bearing function, structural materials, including steel, deform significantly under load. For example, consider a surface wave having a radius of curvature of 2000", with a load of 200 lbs. for 1" radially along the radial wave, bearing against a flat rigid surface. The constant radius of curvature wave will deform so that, if there is no sliding, there will be surface contact over a circumferential length of 0.35". With sliding, the deformations of the surface produce a convergence geometry with much higher full-film load-bearing capacity than would occur if the surfaces were infinitely stiff.
The thrust bearing of the present invention is essentially a wavy disk, or a stack of wavy disks, with the waves radial from the center of the disk and the peak-to-valley wave height from 0.006" to 0.0005". The disks will operate as thrust bearings on either grease or oil, and operate best at least partly submerged in lubricant. A central oil feed provides a good source of lubricant and cooling to the thrust bearing disks when a source of pressure oil is available. Radial oil supply grooves in the low portion of the surface wave assist lubrication.
When particles in the lubricant oil or grease are a problem, it may be highly desirable to have the wavy thrust bearing disk slide against a disk covered with a thin layer (0.030"-0.002") of elastomer. Such an elastomer layer is extremely stiff, and has excellent embeddibility of particles to minimize or eliminate wear, and operates well at low speeds.
The thickness of the thrust bearing disk depends on a number of variables some of which are thrust loads to be carried, the kind of material from which the disk is to be made and the temperatures to be encountered in bearing operation. Variables such as type of lubricant can obviously have a great effect on bearing thickness. It must be remembered that the loads should cause the curvature of the undulations to grow greater and thereby lesser the effective lubricant film to down as low as 0.0001" thickness.
In view of the foregoing it is an object of this invention to provide a thrust bearing in the form of a disk or a round plate having radially extending undulations completely around the surface thereof, said undulations bearing against a rigid planar bearing base with lubricant between the rigid planar bearing base and the undulating surface bearing thereagainst.
It is another object of this invention to provide a hydrodynamic thrust bearing comprising a plurality of disks with radially extending undulations on both surfaces with rigid flat bearing bases interposed therebetween so that every undulating surface bears against a rigid flat bearing surface with lubricant provided between all of said surfaces.
It is yet another object of this invention to provide hydrodynamic thrust bearings as set forth in an earlier object and wherein at least one of the rigid flat bearing surfaces is coated with a thin layer of an elastomer whereby said layer will pick up small foreign particles that may be in the lubricant.
The foregoing and additional objects and advantages will become apparent when taken in connection with the following drawings and detailed description showing by way of example at least one preferred embodiment of the invention.